landy Posted September 10, 2003 Share Posted September 10, 2003 Thirdtap London your answer please?"Sorry the button was red and I just couldn't resist pressing it' Did the button say "don't touch" Got to press it You must not touch itGot to press it You must not touch itGot to press it You must not touch itGot to press it You must not touch itGot to press it You must not touch itGot to press it You must not touch it The voices tell me to go now Link to comment Share on other sites More sharing options...
FatMan Posted March 5, 2004 Share Posted March 5, 2004 Hi I'm building a buzzer for my school quiz too, and I've encountered a couple of snags.The system I'm using involves the use of an SCR. When the buzzer is pressed, the gate current turns on the SCR, a relay connected to the anodes of all the SCRs switches when the SCR turns on, and cuts off the remaining SCRs from the power supply. The Snag: I'm using a 9v battery and a 6v relay. The trouble is that when I set up the circuitry, the SCRs (C106) are already turned on. I can't bill the College if I replace all the SCRs (7!), so while I am kicking myself, turns out that isn't much good. My supplier tells me a relay with a lower rating isn't available. True? Halp! Link to comment Share on other sites More sharing options...
Brian Posted March 5, 2004 Share Posted March 5, 2004 More details please... You have 7 SCRs and 1 relay? How many push-buttons? I'm having trouble visualising the circuit. Link to comment Share on other sites More sharing options...
adam_151 Posted March 5, 2004 Share Posted March 5, 2004 I think I can visulize it, 7 push buttons, each one controls the gate of an scr, as well as feeding leds, the outputs of the scrs are each feed through a diode to latch on a relay which kills the power to the push buttons Adam Link to comment Share on other sites More sharing options...
Brian Posted March 5, 2004 Share Posted March 5, 2004 Ah yes, got it now. Should work, odd that the SCRs come on at power up, have you tied the gates to 0v with a resistor, and fed the gates via a resistor from the pushbuttons? I don't understand the question about lower power relays, the C106 is a 5 Amp part. Link to comment Share on other sites More sharing options...
FatMan Posted March 6, 2004 Share Posted March 6, 2004 The LEDs are in the Anode circuit, i.e. it's Supply>LED>A>K. I've kept the gate floating. I realise now that might be my folly. I'm trying to attach a circuit diagram. Relay up here_________________________________ |...........................| |........................(LED) (Buzzer)....................| |...........................|A |.........................._|_ |........................._\/_(SCR) |______ \___G_/ | ..........switch.........|K ............................| ............................|Gnd (The Dots are there because the spaces werent being taken.)Hope this makes sense.There is something I'm doing wrong here...... When I check the voltage drops, I see that there is a 6V drop across the LED, preventing my 6 V relay from switching. Link to comment Share on other sites More sharing options...
Brian Posted March 7, 2004 Share Posted March 7, 2004 Do you mean something like this... http://www.aux99.co.uk/blue-room/quiz.gif I'm suprised about a 6v drop across the LEDs, way too high. Link to comment Share on other sites More sharing options...
FatMan Posted March 8, 2004 Share Posted March 8, 2004 Pretty much. Only I haven't used resistors other than the ones tying the gates to ground. Or diodes, either. Any ideas regarding the LEDs? They're 10mm LEDs, but I don't see why they should show such a drop. How do you attach images? Link to comment Share on other sites More sharing options...
Brian Posted March 8, 2004 Share Posted March 8, 2004 You should put the series resistor in for the gates, you run the risk of injecting too much current into the junction otherwise. A normal red LED should only drop 2v or so but some big LEDs have a number of seperate chips in series inside. I haven't seen these in 10mm, only 20mm but I guess they might exist, that might explain the 6v drop you are seeing. If you put just an LED in series with a 1k resistor onto the 9v supply does it still drop 6v? The schematic was drawn in a CAD program (circuitmaker) exported as a .WMF file, converted to a .GIF, uploaded to my own webspace and then inseted into my post with the IMG button. Link to comment Share on other sites More sharing options...
FatMan Posted March 8, 2004 Share Posted March 8, 2004 I've put in buzzers in the gate circuits, so I think the need for a 1k is gone (the buzzer resistance is around 600 ohm). Link to comment Share on other sites More sharing options...
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